The proposed research is designed to determine the electrophysiological and/or biochemical mechanisms by which the sensitivity of effector cells gradually adjust to the level of neurotransmitter to which they are exposed. Present evidence suggests that the compensatory increase in sensitivity of smooth muscle cells deprived of their normal contacts with neurotransmitters is related to one or more of the following changes in cellular function: a reduced resting membrane potential, altered binding and/or flux of ions, elevated levels of ATP, increased electrical contact among the cells. The proposed research involves approaching the mechanism(s) by a wide variety of methods including electrophysiological investigations with both microelectrodes and the sucrose-gap apparatus, fluxes of radioactive Ca ion, K plus, Na plus and C1 minus, biochemical determinations of ATP and electron microscopy. All of the results are to be compared with estimates by pharmacological means of the changes in sensitivity of a variety of smooth muscle and cardiac muscle preparations. The temporal development of all changes will receive particularly intensive comparisons. Ultimately we expect to be able to determine the specific changes in cell function which are responsible for postjunctional supersensitivity and subsensitivity. Since it has been suggested by several authors that some types of tolerance and dependence may be manifestations of supersensitivity or subsensitivity, the results of the proposed project may provide some interesting new approaches to investigate the mechanisms underlying such phenomena as tolerance and withdrawal reactions.